Borehole imaging at the time of drilling has been available since the first azimuthal tools were designed and put into use in the 1990s. In water-base muds (WBM), numerous improvements in both the resolution and quality of images have been made, particularly in microresistivity logging tools. The challenge to extend these capabilities to oil-base mud (OBM) has been addressed in a new tool designed to acquire high-resolution images in the logging-while-drilling (LWD) environment. The data are maximized by a dual-physics technique using separate resistivity and ultrasonic imaging sensors.
The new tool is introduced along with the key design features that overcome the challenges associated with LWD imaging in oil-base mud. Sensors are positioned on the rotating drill collar with resulting standoff between the sensors and the formation. For the resistivity image, the tool's electromagnetic signal must pass through this gap which acts as an insulating layer. High-resolution electromagnetic pulses are sent through the mud from the sensor to the formation at multiple frequencies (similar in principle to recently introduced wireline imaging tools for OBM). A novel processing algorithm combines the multiple individual frequencies to produce a robust image across a wide range of formation resistivities. Two sensors are positioned diametrically opposite each other for the resistivity imaging. Four ultrasonic sensors are positioned close to the resistivity sensors. The high-sampling rates and focusing of the sensors deliver a resolution comparable to wireline ultrasonic imaging tools for the ultrasonic images in all mud types (OBM/WBM). Multiple sensors are used for both types of physics measurement, and rapid firing and recording by the sensors maximize full-borehole coverage in the majority of drilling conditions.
An experimental version of the tool has been field tested in a broad variety of drilling and geological environments. To date, over 35,000 ft of data have been acquired in wells ranging from vertical to horizontal. Data acquisition has been in clastics, carbonates, and evaporites having various formation properties. The field test data have confirmed the metrology of both physics types, i.e., resistivity and ultrasonic imaging. Examples are presented demonstrating the range of measurements under different borehole and geological conditions. Results to date have exceeded expectations in terms of imaging capabilities. Moreover, the acquisition of resistivity and ultrasonic images has frequently proved to be complementary, with the resistivity images rich in bedding features and the ultrasonic images more sensitive to fractures and borehole conditions.